Define the primary problems in the establishment of normal mammary epithelial cell lines. The man and mouse are the species to be used. This presentation will focus on the rationale and comparable related work limited to those species. There are but a few cell lines establised from normal mammary epithelial. In the mouse, Owens at al (1974), isolated a mammary epithelial cell line from an outbred strain. Subsequent in vitro transformation cannot be determined by implantation into isologous hosts since this strain is not highly inbred. The cells have retained in vitro many of the in vivo features of transporting epithelia such as apical/basal orientation, occluding tight junctions, and transepithelial transport.